paho_mqtt: Add mqtt compilation files

[zs39]

Summary

It provides the necessary compilation files and configuration management files, and offers a publish demo and a subscribe demo

Impact

Adding a new library has no impact.

Testing

The publish/subscribe functionality was tested locally.

First, start an agent using mosqutto.

Then subscribe to a topic on your PC. The steps are as follows:
mosquitto_sub -h 10.189.144.129 -t "zs39" -v
Then publish it on the Nuttx platform. mqtt_pub -h 10.189.144.129 -t "zs39" -m "your message"

On the PC, you can see the following printout.

zs@zs-ThinkStation-P368 > mosquitto_sub -h 10.189.144.129 -t "zs39" -v
zs39 your message

Then subscribe on the Nuttx client. mqtt_sub -h 10.189.144.129 -t "zs39"

Publish on PC mosquitto_pub -h 10.189.144.129 -t "zs39" -m "hello world" twice

The following can be seen in the NuttX output:

nsh> mqtt_sub -h 10.189.144.129 -t "zs39"
hello world
hello world

[acassis]

@zs39 nice! Please don't forget to add the Documentation at nuttx/Documentation

[linguini1]

Awesome! Can you also open a PR to the NuttX main repository for some documentation about this?